Buried optical fibre pressure sensor for intrusion detection

Optical fibre pressure sensors offer a number of interesting properties for intrusion detection : active sensor, unsensitive to E.-M.I., analog response, potential low cost. We have tested several detection principles bending loss, speckle pattern variation, polarization changes in monomode fibres. For buried sensors, the latter, with ideal fibres, offer a good compromise between the advantages and drawbacks of the two other methods. Actual fibres however are subject to random polarization perturbations which can lead to important signal fading. The effects of these perturbations are analyzed. Extensive investigations have led to a new fibre conditioning and a new relatively simple (electro) optical configuration which should take care of most of the perturbations and, in the same time, appreciably improve the sensitivity (by a factor 10 to 100).